Amorphous and Crystalline Solids: There are three different states of matter around us: solid, liquid, and gas. The solids are hard and are the most frequently used substances. However, there are some solids that are soft as well. Hard solids are referred to as crystalline solids, while soft solids are referred to as amorphous solids. Do you know why some solids are hard and some are soft? To learn more about these solids, read the below article.

Solid State

The solid state of the matter is one in which the constituent particles, such as atoms, molecules, or ions, are tightly packed in the lattice and unable to move. They can only vibrate in the direction of their axes. Solids have distinct features due to the unique packing of constituent particles.

Classification of Solids

Solids are classified into two types based on how their constituent particles, such as atoms, molecules, or ions, are arranged. The two types of solids are:

What is Crystalline Solid?

This is the state of all solid elements (metals and nonmetals) and compounds. A crystalline solid has atoms, molecules, or ions arranged in a highly ordered microscopic structure that forms a crystal lattice that spans in all directions. Common salt \(\left({{\text{NaCl}}} \right),\) sugar (sucrose), diamond, quartz, silver iodide, and other crystalline solids are examples.

Characteristics of Crystalline Solids

In a crystalline solid, the structural elements are organised in a specific order. This distinct pattern repeatedly appears throughout the solid.

A crystalline solid has a particular geometrical shape that is typical of the substance’s nature due to the regular recurrence of this definite pattern. Crystalline solids possess a regular repetition of a definite pattern. This regular pattern extends throughout the three-dimensional network of the crystal.

This is why crystalline solid is said to possess a long-range order. Crystalline solids have distinct melting points and fusion temperatures. Therefore, crystalline solids are regarded as true solids.

When a molten crystalline solid cools, it reverts to its original geometrical configuration. These solids can’t be compressed. Planar surfaces and distinct angles between the faces characterise them. Only definite planes can be cleaved in a crystalline solid, and cleavage produces a clean cut.

Anisotropy Nature of Solids

Anisotropy occurs when a substance exhibits distinct magnitudes of attributes such as electrical conductivity, electrical resistance, refractive index, thermal expansion, and so on, in opposite directions. For example, anisotropic crystalline solids have varying electrical resistance, refractive index, and other properties when measured in different orientations within the same crystal. This is because particles are arranged differently in different directions.

Anisotropy in crystals is due to the different arrangement of particles along with different directions.

For example,
1. In a crystal of silver iodide, the coefficient of thermal expansion is positive in one direction and negative in the other.
2. In sodium chloride crystal \(\left({{\text{NaCl}}} \right),\) the structural units are \({\text{N}}{{\text{a}}^ + }\) and \({\text{C}}{{\text{l}}^ – }\) ions. In the crystal, these units are present at alternate sites. Their arrangement in two dimensions can be shown as follows;

\({\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^{\text{-}}}{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }\)
\({\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^{\text{-}}}{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }\)
\({\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }\)
\({\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }{\text{C}}{{\text{l}}^ – }{\text{N}}{{\text{a}}^ + }\)

Classification of Crystalline Solids

Different forms of crystalline solids may have different types of structural units and different sorts of cohesive forces that keep them together in the crystal. Therefore, the crystalline solids can be categorised into distinct types based on structural units and cohesive forces, as shown below.

1. Molecular Solids
2. Ionic solids
3. Covalent solids
4. Metallic solids

The following table summarises the nature of structural units, cohesive forces that occur between structural units, significant features, and some examples of these different forms of crystalline solids:

Crystal Type	Structural units	Cohesive or bonding forces	Properties	Examples
Molecular solids	The molecules are arranged in a three-dimensional network.	Weak van der Waal’s force	These are often soft, have low melting points, are volatile, are electrical insulators. They are poor heat conductors and have low fusion heat.	Dry ice, ice, iodine, solid argon
Ionic solids	Positive and negative ions are arranged in a three-dimensional network	Strong electrostatic forces	They are hard, brittle, have high melting points, high fusion heat, and are insulators in solid-state, conductors in aqueous solution and molten states.	Salts like \({\text{NaCl}},{\text{KCl}},{\text{Ca}}{{\text{F}}_2},{\text{ZnS}}\)
Covalent solids	Atoms of one or more kinds, bonded together by covalent bonds in a three-dimensional network.	Very strong covalent bond (valence bond)  forces	These are extremely hard, have high melting points, have extremely high fusion heat, and are poor heat and electrical conductors.	Diamond, graphite, Quartz
Metallic solids	Positive ions in a sea of mobile electrons	Metallic bonding	These are hard, malleable and ductile, have fairly high melting points, moderate heat of fusion, good heat and electricity conductors, and metallic lustre.	All metals, e.g., gold, silver, copper, iron, aluminium, zinc, etc., and some alloys

Uses of Crystalline Solids

There are numerous applications for crystalline solids, some of which are as follows:

1. The most attractive example of crystalline solids is diamond, which is often used in the creation of gorgeous jewellery.
2. Quartz is commonly utilised in the production of watches and clocks.
3. In various sectors, crystalline crystals are employed as a raw material.

What is Amorphous Solid?

An amorphous or non-crystalline solid is one that lacks the long-range structure that a crystal possesses. Some examples are rubber, glass, pitch, tar, fused silica, plastics, polymers of high molecular mass, etc.

Characteristics of Amorphous Solid

The structural units of an amorphous solid are not grouped in a defined pattern. As a result, this type of solid lacks clear geometrical forms. There may be an organised arrangement of structural units in an amorphous solid, but it is not very regular and only extends to a few Angstroms.

The uneven pattern is frequently observed beyond this distance. Therefore, the amorphous solids are said to possess only short-range order.

 Properties of Amorphous Solids

1. Amorphous solids do not have sharp melting points. Therefore they liquefy over a wide temperature range.
2. These solids do not possess characteristic heats of fusion.
3. When cut with a knife, these solids do not produce a regular cut.
4. To some extent, amorphous solids can be compressed.
5. In nature, amorphous solids are isotropic, meaning they have the same value for every property in all directions.
6. In an amorphous solid, the refractive index, thermal and electrical conductivities, coefficient of thermal expansion, and other properties are found to be the same regardless of measurement direction.

Uses of Amorphous Solids

In our daily lives, amorphous solids are quite helpful. Below are a few examples of applications;

1. Inorganic glasses are the most frequently used amorphous solids, with applications in building, houseware, laboratory ware, and other areas.
2. Rubber, which is used to make tyres, shoe soles, and other amorphous solids, is another well-known one.

Difference Between Crystalline and Amorphous Solids

1. Arrangement of their Constituent particles: The constituent particles (ions, atoms, or molecules) of a crystalline solid are arranged in a precise geometric pattern in all three dimensions. The order is sufficiently consistent that knowing the arrangement at one site can predict the arrangement at another. This is referred to as long-range order. On the other hand, in an amorphous solid, there is only a limited region where particles are arranged in a regular pattern. This is referred to as a short-range order. As a result, crystalline materials have long-range order, whereas amorphous solids only have short-range order.
2. Melting Points: Melting points is the temperature at which substances melt. Amorphous compounds melt gradually across a temperature range, whereas crystalline solids have sharp melting points.
   As a result, crystalline solids have specific temperatures of fusion, whereas amorphous solids do not.
3. Isotropy and Anisotropy: Amorphous solids are isotropic. This means the Electrical conductivity, refractive index, thermal expansion, and other properties of amorphous solids are the same in all directions, just as in the case of gases or liquids.
   Crystallines solids are anisotropy in nature. This means Anisotropic crystalline solids have varying electrical resistance, refractive index, and other properties when measured in different orientations within the same crystal.
4. Cleavage with the knife: When a crystalline solid is cut with a sharp knife, it cleaves cleanly, but an amorphous solid break apart irregularly.

a) Crystalline solid gives a clean-cut (b) Amorphous solid gives an irregular cut.

5. Heats of Fusion: Amorphous solids do not have a specific heat of fusion, whereas crystalline solids have.
6. Cooling Curves: Amorphous solids have a smooth cooling curve with no breaks, which means that as a molten amorphous solid is cooled, the temperature decreases smoothly over time. The cooling curve of a crystalline solid, on the other hand, is not smooth; there is a break when solidification begins and another after solidification is completed.

Check the tabular form below to understand the difference between amorphous and crystalline solids clearly.

S. No.	Properties	Crystalline solids	Amorphous solids
1.	Arrangement of Structural units	Very regular and extends in three dimensions throughout the crystals (Long-range order)	Not regular throughout the solid (Short-range order)
2.	Geometrical configuration	Definite	Not definite
3.	Melting points	Very sharp	Not sharp
4.	Heat of fusion	Definite and characteristic	Neither definite nor characteristic
5.	Cleavage with a knife	Regular clean cut	Irregular cut
6.	Compressibility	Generally incompressible	Generally compressible to some extent
7.	Nature	True solids	Supercooled liquids or pseudo solids
8.	Anisotropy	Anissotropic in nature	Isotropic in nature

Summary

Solids have distinct features due to the unique packing of constituent particles. Crystalline Solid is a state of all solid elements (metals and non-metals) and compounds. Amorphous Solid is one that lacks the long-range structure that a crystal possesses.

FAQs on Amorphous & Crystalline Solids

Q.1. Which is more stable crystalline or amorphous?
Ans: Crystalline solids are more resistant to chemical degradation than amorphous ones. Hence, crystalline solids are more stable than amorphous solids.

Q.2. Is gold a crystalline solid or amorphous?
Ans: Gold is an element and is a crystalline solid.

Q.3. What are the properties of solids?
Ans: Following are the properties of solids.
1. Arrangement of Structural units
2. Geometrical configuration
3. Melting points
4. Heat of fusion
5. Cleavage with a knife
6. Compressibility
7. Nature
8. Anisotropy

Q.4. Is glass an example of an amorphous solid?
Ans: An amorphous or non-crystalline solid lacks the long-range structure that a crystal possesses. Hence, glass is an example of an amorphous solid.

Q.5. What are crystalline and amorphous solids? Explain with examples?
Ans: A crystalline solid is a solid whose constituents (atoms, molecules, or ions) are arranged in a highly ordered microscopic structure that forms a crystal lattice that extends in all directions. Some examples of crystalline solids are Common salt \(\left({{\text{NaCl}}} \right),\) sugar (sucrose), diamond, quartz, silver iodide, etc. An amorphous or non-crystalline solid lacks the long-range structure that a crystal possesses. Some examples are rubber, glass, pitch, tar, fused silica, plastics, polymers of high molecular mass, etc.

Q.6. Which is better crystalline or amorphous?
Ans: Crystalline solid is better than amorphous solid.